zoate in a carrier of mixed terphenyls. This stream feeds the disproportionation section of the process where the potassium benzoate disproportionates to dipotassium terephthalate under the influence of a zinc oxide catalyst. The product stream is flashed to remove carbon dioxide and benzene, both of which are either re-used or recovered as by-products. The catalyst is recycled as are other re-usable materials. Solid waste is removed as a carbonaceous residue from the catalyst recovery unit. The cost estimates that Marwil provides apply to a 220 million lb-per-year plant based on this process. A 5 centper-lb credit for by-product benzene is considered enough to give the process at least a parity with the currently used oxidation process and maybe as much as a 2 cent-per-lb advantage depending on the design assumptions made. If the cost estimates are realized, the PRP process will produce a transfer price of about 20.2 cents per lb for TPA, which is about 8 cents per lb less than TPA made from ρ-xylene via oxidation. However, Marwil notes that the process economics are very sensitive to the prices of both toluene and ρ-xylene. Phillips believes that its new process will remain at least competitive because both toluene and ρ-xylene are expected to escalate by about the same amounts. Another valuable raw material and in termediate is synthetic phenol. According to Arthur S. Nislick, of C-E Lummus Corp., three of the four major processes yielding phenol today are based on ben zene and the other is based on liquidphase oxidation of toluene. About 90% of world phenol production comes from cu-
Higher feedstock, energy prices prompt new petrochemical processes Changing feedstock cost patterns—current or potential—and higher energy costs are prompting a new look at petrochemical technology. As a result, a spate of new processes is starting to make the scene. Last week's 85th national meeting of the American Institute of Chemical Engineers in Philadelphia provided a sampling of such new processes. Among them: a process for terephthalic acid starting with toluene, a vapor-phase oxidation route to phenol from benzoic acid, and a new system of reactions to make higher olefins from ethylene. The terephthalic acid (TPA) process is a low-cost route developed jointly by Phillips Petroleum and Rhône-Poulenc Industries. It begins with toluene instead of the more expensive ρ-xylene, and is a modification of the Henkel II process developed in Germany about 30 years ago. TPA has been challenging dimethyl terephthalate as the principal raw mate rial for polyester for some time. But the P R P process likely will have to wait awhile for a try, although it has been coaxed through R&D stages and is now being considered for full commercial-scale operation. S. J. Marwil of Phillips' Bartlesville R&D laboratories notes that world fiber markets have been in a slump, with plants operating at 70 to 80% of capacity. This means that no new raw material plants likely will be built in the near future, even with what Marwil considers as attractive
a process as PRP. However, if the trends reverse, as they are expected to do even tually, then Phillips and Rhône-Poulenc are ready. In the Henkel II process, potassium benzoate reacts with carbon dioxide over cadmium or zinc catalysts to form dipotassium terephthalate and benzene. Acidification of the dipotassium terephthalate frees TPA. Although attractive from a process viewpoint, the Henkel II process does present some problems that have severely limited its use. In particular, Marwil says, the change from solids to liquids and back to solids again in the course of the process results in inefficient heat transfer, reactor plugging, consequent low conversions, and low catalyst selectivity, as well as the problem of disposing of strong acid residua. The improvements in the PRP process, claims Marwil, have diminished, if not removed, these problems. The P R P process begins with the oxidation of toluene to benzoic acid at moderate conditions over cobalt catalysts. Benzoic acid yield is from 90 to 92%. The reactor effluent is distilled to yield 99.5% benzoic acid, which is mixed with recycled dipotassium terephthalate to form terephthalic acid and potassium benzoate. Crude TPA precipitates out and is further refined downstream to fiber grade. The liquid phase, containing unreacted potassium benzoate, benzoic acid, and water, goes to a feed preparation section, which yields a slurry of potassium ben-
PRP process converts toluene to terephthalic acid
Toluene
Oxidation
Benzoic acid
Acidification
Potassium benzoate
Crude terephthalic acid
Purification
Fiber-grade terephthalic • acid
Potassium benzoate
Dipotassium terephthalate Methyl l*| terephthalate removal
Feed preparation
| Φ _
Catalyst preparation
._.„.._,_„. ,..__,,.,f^,^....,„„.. «„«*> Carbon f dioxide
Disproportionation
Zinc oxide Methyl terephthalate + dipotassium terephthalate, potassium benzoate
34
C&EN June 12, 1978
Benzene re
